The present invention relates to a desalination method and a desalination apparatus that are used for desalinating raw water (e.g. saline water, sea water) by utilizing a local unused waste heat source, or relates to a desalination method and a desalination apparatus that produce fresh water or suitable water, drinking water or demineralized water by softening hard water, for example.
In various facilities, e.g. plants and marine structures, agricultural land, in inland areas, islands and desert regions, it is difficult to obtain suitable water for industrial use, drinking water or agricultural water, and in many cases, it is necessary to transport water by ship or truck or to lay pipelines. In other cases, these facilities use, a membrane or other types of desalination apparatus that necessarily consume a large amount of electric energy.
While thermal or nuclear power plants and so forth reuse high-temperature waste heat to generate electricity from steam turbines, low-pressure steam and its potential heat energy is discharged as waste. In fact, various kinds of low-temperature heat exist, for example, local (regional) temperature differences, terrestrial heat, and fermentation heat.
Many plants or other facilities having a waste heat source as stated above require demineralized water or water having a low impurity content to operate. The technique of transporting such water by ship, truck or pipelines as stated above suffers from the problem that the cost of transportation, the construction costs and maintenance and management costs are high. Membranes or other types of desalination apparatus consume a large amount of electric energy, and this also contributes to high operation costs.
In order to efficiently desalinate saline or hard water, vacuum evaporation type desalination apparatuses have been proposed.
As conventional vacuum evaporation type desalination apparatuses in particular, those which use a flash system or a multiple-effect can system have been proposed.
However, conventional flash or multiple-effect desalination apparatuses suffer from the disadvantage that the amount of water used for cooling in condensers is large, and the amount of water discharged is correspondingly large, resulting in the need for a great amount of pump power.
In a flash system, if the difference in temperature between a heat source used and cooling water is small, efficiency becomes low, and it is difficult to realize an efficient multiple-effect system. Similarly, the conventional multiple-effect can system suffers from the disadvantage that if a temperature difference between a heat source used and cooling water is small, it is impossible to increase the number of cans used to form a multiple-effect desalination apparatus. Accordingly, it is difficult to improve efficiency. Consequently, in a case where a temperature difference between a heat source used and cooling water is small, it is necessary to increase the heat transfer surface area of the apparatus. This causes a rise in installation costs and necessitates the use of a large installation area.
Furthermore, because both the systems use a continuous operation mode, it is necessary to run a fluid transfer pump, a vacuum pump, etc. at all times. Consequently, the amount of power consumed in the whole system is large, and efficiency is low. Regarding a vacuum pump in particular, because a high degree of vacuum is produced in a low-temperature condition, a large amount of water vapor is entrained in the extracted gas. Therefore, a large capacity vacuum pump which consumes a large amount of power is required.
The present invention was made in view of the above-described circumstances, and it is an object of the present invention to provide a desalination method and a desalination apparatus capable of producing fresh water stably at reduced cost by making cascade use of low-temperature waste heat as an energy source, which has heretofore been disposed of without being used.
To solve the above-described problems, according to a first aspect of the invention, a desalination method for raw water comprising the steps of: (1) supplying raw water into a confined space means; (2) evacuating the confined space means and depressurizing an inside thereof; (3) supplying low-temperature waste heat into the confined space means so as to subject the low-temperature waste heat and the raw water in the confined space means to heat exchange and generate water vapor in the confined space means; and (4) cooling the water vapor to obtain a distilled water.
The confined space means may comprise a single evaporation can.
Instead, the confined space means may comprise a plurality of evaporation cans which are connected in series, wherein the low-temperature waste heat is supplied into a first evaporation can, and wherein in each pair of adjacent evaporation cans, the downstream-side evaporation can receives water vapor from an upstream-side evaporation can, cools the water vapor with raw water in the downstream-side evaporation can and thereby produces distilled water, and also heats the raw water in the downstream-side evaporation can and generates water vapor.
The confined space means may comprises a plurality of evaporation cans which are connected in parallel rows, wherein the steps of (1) to (4) above are switched over from one evaporation can to another to thereby enable a continuous desalinating operation.
In the method described above, the step of evacuating may be effected intermittently or at optional time, for example, for a predetermined period during the time of starting the desalinating operation.
The step of supplying raw water into the confined space means could be effected by evacuating the confined space means while opening the confined space means to a raw water source. The method further may comprise a step of discharging concentrated raw water out of the confined space means and this step may be effected, after opening the confined space means to the atmosphere, by opening the confined space means and allowing flowing down of the concentrated raw water therefrom.
According to a second aspect of the invention, a desalination apparatus includes a heat exchanger cooperating with an evaporation can so as to subject low-temperature waste heat and raw water in the evaporation can to heat exchange and generate water vapor in the evaporation can; a condenser cooperating with a raw water tank to receive the water vapor from the evaporation can, cool the water vapor by subjecting the water vapor and raw water in the raw water tank to heat exchange and obtain distilled water; a distilled water tank for storing the distilled water; vacuum means for evacuating the evaporation can and depressurizing the inside thereof so as to promote generation of water vapor in the evaporation can; and raw water supply means for supplying raw water to the evaporation can.
According to a further aspect of the invention, a desalination apparatus includes a heat exchanger cooperating with an evaporation can so as to subject low-temperature waste heat and raw water in the evaporation can to heat exchange and generate water vapor in the evaporation can; a condenser adapted to receive the water vapor from the evaporation can, cool the water vapor by subjecting the water vapor and cooling water to heat exchange and obtain distilled water; a distilled water tank for storing the distilled water; vacuum means for evacuating the evaporation can and depressurizing the inside thereof so as to promote generation of water vapor in the evaporation can; and raw water supply means for supplying raw water to the evaporation can.
In the desalination apparatus described above, the low-temperature waste heat may consist of the potential heat of exhaust steam from a steam turbine for electric power generation in a plant.
The desalination apparatus may be incorporated in series and/or parallel with a condenser of the steam turbine for electric power generation, or can be used in place of the condenser. In a case where the desalination apparatus is used in place of the condenser, it is not easy for only one set of desalination apparatus to treat a large amount of exhaust steam as discharged from an electric power plant. Therefore, a plurality of desalination apparatuses may be used, by being incorporated in parallel.
In the desalination apparatus stated above, a plurality of evaporation cans may be provided, wherein the heat exchanger is arranged to cooperate with a first evaporation can, and the condenser is arranged such that in each pair of adjacent evaporation cans, the downstream-side evaporation can receives water vapor from an upstream-side evaporation can, cools the water vapor with raw water in a downstream-side evaporation can and thereby produces distilled water, and also heats the raw water in the downstream-side evaporation can and generates water vapor.
The desalination, apparatus may include control means for controlling the operation of the vacuum means and opening and closing of a control valve connected to the evacuation can.
The control means may control the vacuum means and the control valve so that evacuation of the evaporation can and opening of the evaporation can to the atmosphere are intermittently repeated, thereby enabling a batch (intermittent) operation.
In the desalination apparatus stated above, a plurality of evaporation cans may be disposed in parallel rows each consisting of at least one evaporation can, and the control means may control the vacuum means and the control valve so that the evaporation cans in the rows are not simultaneously opened to the atmosphere, thereby enabling a continuous operation of the desalination apparatus. Such continuous operation may be either a rated operation or an operation following variations in waste heat (heat source).
In the desalination apparatus stated above, the raw water supply means may be formed by vacuum means and a control valve connected to the evaporation can which is opened or closed by the control means.
The desalination apparatus may further include concentrated raw water or waste brine discharge means for discharging concentrated raw water from the evaporation can. The concentrated raw water discharge means may be formed by a control valve connected to a lower part of the evaporation can that is opened or closed by the control means.
In the desalination apparatus stated above, the evaporation can, heat exchanger, condenser, distilled water tank, vacuum means and raw water supply means may be unitized in a single frame. In the case that the desalination apparatus further includes a concentrated raw water discharge means, it is possible to further incorporate a concentrated raw water discharge means in a single frame.
A plurality of desalination apparatus thus unitized in a single frame may be further assembled in a single unit.
Since the desalination apparatus of the present invention uses a low-temperature waste heat source that has heretofore been unused as a heat source, for example, low-temperature waste heat from a thermal or nuclear power plant, it is, therefore, possible to effectively use such heat, which has heretofore been disposed of.
In addition, it is possible to ensure extremely stable supply of a heat source and a cooling source and to perform stable desalination by utilizing low-temperature waste heat discharged from a thermal or nuclear power plant as a heat source and also by utilizing sea water as a cooling source for the condenser in the final stage, making use of the conditions of location of thermal or nuclear power plants, which are installed by the sea in view of the transport of a power generating fuel and ensuring a place where waste heat is disposed of (i.e. heat radiating source).
Further, the cascade use of a low-temperature waste heat source, which is an energy source heretofore unused, makes it possible to suppress the use of fossil fuels and to minimize the generation of global warming gases. Thus, it is possible to provide a resource-recovering technique by desalination that is friendly to the global environment.
According to the present invention, raw water is stored in an evaporation can, the inside of the evaporation can is depressurized by a vacuum pump, and the raw water is heated with low-temperature waste heat. This enables evaporation to be promoted at a low temperature. Accordingly, it is possible to effect desalination by using low-temperature waste heat produced by an electric power plant or the like.
In addition, since evaporation can be carried out at a low-temperature, even if the temperature difference between a heat source used and cooling water is small, the evaporation efficiency can be improved by employing a multiple-effect evaporation can system and repeatedly using heat.
Furthermore, because raw water stored in a can is evaporated, an intermittent or batch operation can be performed. Accordingly, power consumption can be reduced markedly. In particular, if the apparatus is operated by a batch mode and if the inside of the apparatus is evacuated at the time of starting the operation, and the inside of the apparatus is opened to the atmosphere upon the termination of the operation, it is possible to supply raw water into the apparatus and to discharge concentrated water out of the apparatus by actuating and stopping operation of the vacuum means. Therefore, the apparatus can basically be operated by the operation of the vacuum pump alone. Accordingly, power consumption can be further reduced.
Furthermore, a continuous operation can be performed by operating in batch mode a plurality of apparatuses arranged in parallel by changing them over from one to another.
Furthermore, because the apparatus can be operated with high efficiency and with low power, the whole apparatus can be unitized in a compact unit. Accordingly, it is possible to make transportation, carrying in and installation of the apparatus easy.
The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative examples.